For portable electronic devices, such as cell phones, personal digital assistants, notebook computers, camcorders, and handheld game machines, advanced functionality results in an increased number of heat-generating components mounted on a board. In use of the portable electronic device, heat dissipation from heat-generating components might cause the package surface to be locally heated to high temperature.
Patent Document 1 discloses a structure in which heat from heat-generating components housed in the package of a portable electronic device is advantageously diffused by a heat-conducting member with superior heat dissipation performance which is arranged around the heat-generating components. Examples of the heat-conducting member include metallic materials such as aluminum alloy, magnesium alloy, stainless steel, and plain steel.
Patent Document 2 discloses a flame-retardant resin composition which includes aromatic polycarbonate having a phosphorous flame retardant added thereto and is used as a material for packages and components of electronic devices.
Patent Document 3 discloses a resin composition including a heat-absorbing material such as aluminum hydroxide or magnesium hydroxide. Aluminum hydroxide and magnesium hydroxide reduce combustion heat through an endothermic reaction. The endothermic reaction is a reaction in which water is released. The endothermic reaction exerts a flame retardant effect.
Patent Document 4 discloses a battery pack including an exterior case, batteries housed in the exterior case, and a heat-insulating layer formed between the inner surface of the exterior case and the batteries. Patent Document 4 also discloses that such a heat-insulating layer prevents reduction in battery performance due to lowered environmental temperature.    Patent Document 1: International Publication WO2008/062879 pamphlet    Patent Document 2: Specification of Japanese Patent No. 3682148    Patent Document 3: Specification of Japanese Patent No. 3408676    Patent Document 4: Japanese Laid-Open Patent Publication. No. Hei 5-234573
Currently, among secondary batteries widely used as power sources of portable electronic devices, lithium ion secondary batteries are mainly used in which the positive electrode includes lithium cobalt oxide and the negative electrode includes graphite. It is assumed that if such a lithium ion secondary battery is subjected to an extremely significant impact, gas or flame goes out of the battery.
On the other hand, to achieve higher-capacity and higher-energy density lithium ion secondary batteries, attempts have been made to use silicon- or tin-based active materials as negative electrode active materials. In particular, the silicon-based active material has an extremely high capacity compared to graphite. It is assumed that if a lithium ion secondary battery using a silicon-based active material is subjected to an extremely significant impact, an alkali salt such as lithium orthosilicate melts to become a high-temperature liquid, which flows out of the battery.